Membrane-impermeable macromolecules such as peptides, proteins, oligonucleotides, DNA, RNA, and a variety of other probes can alter or assay cell function. Among available methods for introducing molecules into embryos or cells, such as chemical (ATP, EDTA), vehicular (erythrocyte fusion, vesicle fusion), electrical (electroporation), and mechanical (microinjection, hyposmotic shock, sonication or microprojectiles), microinjection is the standard method for loading embryos and cells. It can reproducibly deliver large numbers of macromolecules to most embryo and cell types with high viability and function. Minimally invasive microinjection and surgical tools with integrated sensors are critical for a wide range of studies in biology and medicine, including calibrated trans-membrane delivery of genetic material into biological model systems, such as Drosophila embryos, to enable screening of gene functions.
The knowledge created by genome sequencing has brought unprecedented opportunities to further study the genetic and molecular mechanisms of development and disease. The genome sequence of the fruit fly, (Drosophila melanogaster) has enabled systematic studies of the functions of the approximately 13,600 Drosophila genes. A powerful technique for learning about gene functions is RNA-interference (RNAi) through microinjections. In RNAi experiments, specific genes are silenced by the presence of dsRNA (double-stranded RNA). An observed change in phenotype indicates the function of the silenced gene. Typically, 100-200 fly embryos per assessed gene are injected during the first 60 minutes of their development, each with 60 picoliters of dsRNA. However, common manual injection involves injecting embryos and cells one at a time with individual glass micropipettes observed under a microscope, which is extremely labor intensive.
Localized and accurate microinjection of genetic material into biological model systems, such as Drosophila, is desirable for a variety of studies of developmental biology and genetics. For such studies to be carried out in vivo, the damage caused by the injection must be minimized. Reducing the penetration force is desirable for microinjection systems for biology and genetics studies, such as RNAi for gene silencing. These and other issues continue to present challenges to microsurgical tools and, in particular, to microinjectors for RNAi gene silencing.